1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically, to a termination region of a power semiconductor device with a trench gate structure.
2. Description of the Background Art
Power semiconductor devices have been applied as semiconductor devices in inverter circuits and converter circuits for motor control in response to industrial needs. These power semiconductor devices have also been applied frequently in recent years in wind power generation and photovoltaic power generation systems in response to higher awareness of ecological issues.
High-speed switching performance and reduction in on-resistance have been required especially for power MOSFETs applied in power supply circuits and switching power supply circuits.
As a technical trend for reduction in on-resistance of power MOSFETs, patterns have been made finer in wafer processing to increase the density of transistor cells. A trench gate structure has been employed to increase the density of transistor cells (see Japanese Patent No. 3904648).
Reduction in gate capacitance has been required to realize high-speed switching. The opening width of a trench should be made smaller, or a trench should be made shallower for reduction in gate capacitance. Reduction in gate capacitance is also achieved by using stripe trenches in addition to mesh trenches.
A fine processing technique using an excimer stepper has been employed to make a trench pattern finer. Meanwhile, an end portion of a trench becomes sharp pointed with a smaller width of a pattern.
A gate oxide film cannot be made uniform if it is formed in a trench with such a sharp pointed end portion, leading to reduction in dielectric strength.
A strong electric field is applied to the end portion of a trench if a voltage is applied between a source and a drain with a gate and the source short circuited, leading to unstable condition caused for example by oscillation of a breakdown voltage.